Si=P double bonds: Experimental and theoretical study of an NHC-stabilized phosphasilenylidene

Article abstract of DOI:10.1002/anie.201411264

An experimental and theoretical study of the first compound featuring a Si=P bond to a two-coordinate silicon atom is reported. The NHC-stabilized phosphasilenylidene (IDipp)Si=PMes(IDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene, Mes= 2,4,6-tBu₃C₆H₂) was prepared by SiMe₃Cl elimination from SiCl₂(IDipp) and LiP-(Mes)SiMe₃ and characterized by X-ray crystallography, NMR spectroscopy, cyclic voltammetry, and UV/Vis spectroscopy. It has a planar trans-bent geometry with a short Si-P distance of 2.1188(7) ? and acute bonding angles at Si (96.90(6)°) and P (95.38(6)°). The bonding parameters indicate the presence of a Si=P bond with a lone electron pair of high s-character at Si and P, in agreement with natural bond orbital (NBO) analysis. Comparative cyclic voltammetric and UV/Vis spectroscopic experiments of this compound, the disilicon(0) compound (IDipp)Si=Si(IDipp), and the diphosphene MesP=PMesreveal, in combination with quantum chemical calculations, the isolobal relationship of the three double-bond systems.

Full text of DOI:10.1002/anie.201411264

Angewandte
Chemie
DOI: 10.1002/anie.201411264
Main-Group Multiple Bonds
=
Si P Double Bonds: Experimental and Theoretical Study of a NHC-
Stabilized Phosphasilenylidene**
Daniel Geiß, Marius I. Arz, Martin Straßmann, Gregor Schnakenburg, and
Alexander C. Filippou*
Abstract: An experimental and theoretical study of the first
compound featuring a Si=P bond to a two-coordinate silicon
atom is reported. The NHC-stabilized phosphasilenylidene
isophosphaalkynes (B, Figure 1), are still elusive owing to
[3,4]
their high thermodynamic and kinetic instability, which can
be rationalized with the reluctance of phosphorus and the
[5]
(
IDipp)Si=PMes* (IDipp = 1,3-bis(2,6-diisopropylphenyl)-
other 3p-block elements towards isovalent s/p hybridization.
imidazolin-2-ylidene, Mes* = 2,4,6-tBu C H ) was prepared
Consequently, the valence-isoelectronic silicon analogues of
phosphaalkynes (C and D in Figure 1) are expected to be
highly reactive species. In fact, attempts to generate phos-
phasilynes (silylidynephosphanes) C from suitable precursors
failed so far. Quantum chemical calculations revealed that
the relative stability of phosphasilynes C versus their constitu-
tional isomers D (phosphasilenylidenes) correlates with the
electronegativity of the substituent R. Electronegative sub-
3
6
2
by SiMe Cl elimination from SiCl (IDipp) and LiP-
3
2
(
Mes*)SiMe3 and characterized by X-ray crystallography,
NMR spectroscopy, cyclic voltammetry, and UV/Vis spectros-
[
6]
copy. It has a planar trans-bent geometry with a short SiꢀP
distance of 2.1188(7) ꢀ and acute bonding angles at Si
(
96.90(6)8) and P (95.38(6)8). The bonding parameters
indicate the presence of a Si=P bond with a lone electron
pair of high s-character at Si and P, in agreement with natural
bond orbital (NBO) analysis. Comparative cyclic voltammet-
ric and UV/Vis spectroscopic experiments of this compound,
the disilicon(0) compound (IDipp)Si=Si(IDipp), and the
diphosphene Mes*P=PMes* reveal, in combination with
stituents, such as F, OH, OMe, NH , Me, Ph, stabilize the SiꢀP
2
triple bond in the linear isomer C, whereas electropositive
substituents, such as R = Li, BeH, BH , H, SiH , favor the
2
3
bent isomer D featuring a SiꢀP double bond and a lone pair of
[
6,7]
electrons at silicon.
Remarkably, the parent compound
quantum chemical calculations, the isolobal relationship of
the three double-bond systems.
SiPH was recently generated by electric discharge of SiH4/
PH or SiH /P /H mixtures and shown by FT microwave and
3
4
4
2
millimeter wave absorption spectroscopy to have a bent
P
hosphaalkynes (A, Figure 1) are versatile building blocks
in organoelement chemistry. Their chemistry evolved rapidly
structure (D, Figure 1) with a SiꢀP double bond, a PꢀH single
bond and an acute Si-P-H angle of 60.58, pointing to
a significant interaction of the PꢀH bond with the Si center,
after the isolation of a derivative that is stable at room
[
1,2]
[8,9]
temperature (tBuCꢁP) by Becker et al. in 1981.
In
as predicted previously by quantum chemical calculations.
comparison, the valence isomers of phosphaalkynes, the
However, attempts to prepare phosphasilenylidenes (DSi=
PR), which are stable in solution or in the solid state have not
been reported to date.
Recently, N-heterocyclic carbenes (NHCs) have been
shown to be particularly useful Lewis bases allowing the
stabilization of highly reactive, unsaturated Si species such as
[10]
[11]
[12] [13]
Si2,
SiX (X = Cl,
Br,
I
), SiClR (R = 2,6-Ar C H
2 6 3
2
(
Ar= 2,4,6-Me C H ,
2,4,6-iPr C H ),
N(SiMe )(2,6-
3
6
2
3
6
2
3
[
14,15]
+ [13]
[16]
iPr C H )),
Group 14 homologue of a vinylidene.
SiI , a Si atom, or R Si=GeD, a heavier
[17]
2
6
3
2
Based on these
Figure 1. Constitutional isomers of REP (E=C, Si). The lone electron
results we envisaged that NHCs might be also suitable to trap
pairs are indicated by two dots.
phosphasilenylidenes (DSi=PR), and we thus decided to use
the 1,2-elimination methodology of SiMe Cl, which proved to
3
be particularly successful in the formation of C=P, CꢁP, or
[
2a,e,f]
P=P bonds.
Herein, we present the successful implemen-
II
[*] D. Geiß, M. I. Arz, M. Straßmann, Dr. G. Schnakenburg,
tation of this strategy into Si chemistry with the synthesis and
full characterization of a room-temperature stable NHC-
stabilized phosphasilenylidene.
Prof. Dr. A. C. Filippou
Institut fꢀr Anorganische Chemie, Universitꢁt Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
E-mail: filippou@uni-bonn.de
SiCl (IDipp) (IDipp = 1,3-bis(2,6-diisopropylphenyl)imi-
2
[11]
dazolin-2-ylidene)
tBu C H ; TMS = SiMe ) were chosen as promising starting
and LiP(Mes*)(TMS) (Mes* = 2,4,6-
[
**] We thank the Deutsche Forschungsgemeinschaft (SFB 813) for the
generous financial support of this work. We also thank C. Schmidt,
K. Prochnicki, H. Spitz, Dr. W. Hoffbauer, and Dr. B. Lewall for their
contribution to the experimental studies.
[18]
3
6
2
3
materials to test the 1,2-elimination. Addition of one equiv-
alent of LiP(Mes*)(TMS) to a yellow solution of SiCl (IDipp)
2
in fluorobenzene at ꢀ308C (Scheme 1) was accompanied by
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411264.
a rapid color change to deep red. After warming to ambient
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Figure 2. DIAMOND plot of the molecular structure of 1·Et O at 123 K
2
[
42]
in the solid state. Ellipsoids are set at 30% probability; hydrogen
atoms and the solvent are omitted for clarity. Selected bond lengths
Scheme 1. Synthesis of (IDipp)Si=PMes* (1) via the assumed inter-
mediate 2. Lone pair of electrons are represented by two dots; formal
charges are encircled.
[
ꢂ], angles [8], and torsion angles [8]: Si–P 2.1188(7), Si–C1 1.960(2),
P–C28 1.877(2); P-Si-C1 96.90(6), Si-P-C28 95.38(6), C1-Si-P-C28
1
8
78.10(7), N1-C1-Si-P 92.7(2), N2-C1-Si-P ꢀ103.9(1), Si-P-C28-C29
1.4(1), Si-P-C28-C33 ꢀ91.7(1).
temperature, a second color change from deep red to brown–
red was observed with concomitant precipitation of a white
3
1
1
solid (LiCl). P{ H} NMR analysis of the resulting reaction
solution revealed the formation of the NHC-stabilized
agreement with the results of the natural bond orbital (NBO)
analysis of 1 (see below). The Si=P bond length of
1 (2.1188(7) ꢀ) compares well with the mean value
[
19]
phosphasilenylidene 1 along with PH(Mes*)(TMS)
a small amount of P(Mes*)(TMS)2.
purified by fractional crystallization from n-hexane and
isolated as a bright orange, very air-sensitive solid in 39%
yield. Compound 1 is stable at ambient temperature under
exclusion of air and moisture for several months.
and
[20]
Compound 1 was
(2.1315(1) ꢀ) of the Si=Si bond length of Si (IDipp)
2
2
[10]
(2.229(1) ꢀ)
and the P=P bond length of P Mes*
2
2
[
24]
(2.034(2) ꢀ), and lies in the range of the Si=P bond lengths
[
21]
of phosphasilenes (silylidenephosphanes) (R Si=PR: d(Si=
2
[25]
P) = 2.062(1)–2.172(1) ꢀ). Notably, the Mes* and the NHC
substituents are oriented almost orthogonally to the central
core, with dihedral angles of 92.7(2)8 (N1-C1-Si-P) and
81.4(1)8 (Si-P-C28-C29). The same conformation was
The course of the reaction leading to 1 was followed by
3
1
1
29
1
P{ H} and Si{ H} NMR spectroscopy at low temperature.
3
1
1
29
1
The P{ H} and Si{ H} spectra of the deep red fluoroben-
zene solution obtained at ꢀ308C revealed the formation of an
observed in Si (Idipp)2 (N-C-Si-Si# 90.88), whereas in
2
3
1
intermediate displaying one P singlet at d = ꢀ117.3 ppm
P Mes* the Mes* substituents adopt a twisted conformation
2
2
2
9
1
[24,26]
flanked by two pairs of Si satellite signals ( J(P,Si) = 232 Hz
(C-C-P-P# 61.58).
Salient spectroscopic features of 1 are the very deshielded
1
29
and J(P,Si) = 46 Hz), and two Si doublets at d = ꢀ9.8 ppm
J(P,Si) = 232 Hz) and + 1.6 ppm ( J(P,Si) = 46 Hz), respec-
1
1
29
31
29
1
(
Si and P nuclei. In fact, the Si{ H} NMR spectrum of 1 in
[
21]
1
tively.
This intermediate is suggested to be the NHC-
C D displays a doublet signal at d = 267.3 ppm ( J(P,Si) =
6 6
stabilized phosphinosilylene 2 based on a comparison of its
NMR features with those of the base-stabilized silicon(II)
phosphanides (phosphinosilylenes) (PhC(NtBu) )SiP(TMS)
170.4 Hz), which appears at even lower field than that of
2
9
[10]
Si (IDipp)2 (d( Si) = 224.5 ppm in C D )
deshielded Si NMR signal of a phosphasilene (d( Si) of
or the most
2
6
6
2
9
29
2
2
1
[27,28]
(
(
(
d
=+ 44 ppm ( J(P,Si) = 194 Hz); dSi(TMS) =+ 3.1 ppm
(tBu Si)(Trip)Si=PH (E isomer) = 249.8 ppm).
The large
SiP
3
1
[22]
1
J(P,Si) = 22.9 Hz))
IiPr Me ) (R = 2,6-(2,4,6-Me C H ) C H ,
and Si[P(H)(R)][N(Dipp)(TMS)]-
Dipp = 2,6-
J(P,Si) coupling constant of 170.4 Hz is indicative of Si=P
[
27,28]
bonds
and considerably larger than those of silyl phos-
2
2
3
6
2
2
6
3
[29]
31
1
iPr C H , IiPr Me = 1,3-diisopropyl-4,5-dimethylimidazolin-
phanes. Similarly, the P{ H} NMR spectrum of 1 in C D
2
6
3
2
2
6 6
1
[23]
2
-ylidene; d = ꢀ8.8 ppm ( J(P,Si) = 145.4 Hz)).
Com-
shows a strongly deshielded singlet signal at d = 402.4 ppm
Si
2
9
1
31
pound 2 eliminates TMSCl upon warming to ambient
temperature to give 1 (Scheme 1).
with Si satellites ( J(P,Si) = 170.4 Hz, 4.9%). The P NMR
signal of 1 appears at even lower field than the most
3
1
The molecular structure of 1·Et O was determined by
deshielded P NMR signal observed so far for a phosphasi-
lene (d( P) of (tBu MeSi) Si=PMes* in C D = 389.3 ppm,
J(P,Si) = 171.3 Hz),
diphosphene P Mes* (d( P) in C D = 494.2 ppm).
2 2 6 6
2
3
1
single-crystal X-ray diffraction (Figure 2). Compound 1 fea-
2
2
6
6
[10]
[24]
1
[28j,30]
tures as its isolobal congeners Si (IDipp)2 and P Mes*2
but at higher field than that of the
2
2
3
1
[31]
a trans-bent planar geometry with a torsion angle C1-Si-P-
C28 of 178.10(7)8. The angles at the Si (P-Si-C1 96.90(6)8),
and the P atom (Si-P-C28 95.38(6)8) resemble those of
3
1
The P chemical shift of 1 in solution (dsoln = 402.4 ppm)
compares well with the isotropic value in the solid state
[
10]
31
Si (IDipp)2 (Si-Si-C 93.37(5)8)
1
and P Mes*2 (P-P-C
(d( P) = 398.3 ppm), suggesting a minor influence of inter-
2
2
iso
[
24]
02.8(1)8), respectively. These angles indicate that silicon
molecular or conformational effects on the chemical shift
[21]
and phosphorus use predominantly p-orbitals for the Si=P
bond in 1 and suggest furthermore the presence of a lone
electron pair with high s character at each atom, in full
(Table 1). The chemical shift tensor components d derived
ii
3
1
1
from the solid-state MAS P{ H} spectrum of 1 reveal a large
chemical shift anisotropy with a span Dd (d ꢀd ) of
1
1
33
2
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Chemie
3
1
[a]
Table 1: Experimental and calculated P NMR spectroscopic data of 1.
properties, shape, and approximate
energy of the frontier orbitals
[
d]
[e]
Compound
Method
d11
d22
d33
diso
dsoln
(
LUMO, HOMO, and HOMOꢀ1),
[
b]
c]
[b]
[b]
[c]
MAS-NMR
B3LYP/6-311G**
MAS-NMR
1252
31.2
ꢀ88.4
ꢀ58.2
ꢀ3
398.3
480.7
494
402.4
494.2
(
IDipp)Si=PMes* (1)
confirming the isolobal linkage
between these three double-bond
systems.
cases the symmetric combination of
the lone-pair orbitals (n ), the
[
[c]
1357
1236
143.2
249
Mes*P=PMes*
[
21,35]
The HOMO is in all
[
33b]
[
a] The experimental values of P Mes* are listed for comparison.
vs. 85% aqueous H PO solution (d=0 ppm). [b] The principal components of the chemical shift tensor
d , d , and d ) were obtained by analysis of the side band intensities of the solid-state MAS P NMR
spectrum of 1. [c] The chemical shielding tensor components s , s , and s of 1 were calculated at the
Chemical shifts are given in ppm
2
2
3
4
3
1
+
(
11 22 33
HOMOꢀ1 is the EꢀE (E = Si, P)
1
1
22
33
B3LYP/6-311G** level of theory and converted into the chemical shift tensor components d , d , and p bonding orbital, and the LUMO is
1
1
22
d33 using the equations d (1)=(s (PMe )ꢀs (1))+d(PMe ), where s (PMe )=s (PMe )=
the EꢀE p* orbital (Figure 3).
ll
ii
3
ii
3
11
3
22
3
3
1
s (PMe )=357 ppm calculated at the same level of theory, and d(PMe ) is the experimental
P
3
3
3
3
Comparison of the frontier orbi-
tals reveals a shift of the HOMO to
higher energy in the series Mes*P=
PMes*!(IDipp)Si=PMes* (1)!
chemical shift of PMe in solution (d(PMe ) in C D at 298 K=ꢀ61.9 ppm. [d] d =(d11 +d22 +d )/3.
3
3
6
6
iso
33
3
1
[
e] P NMR chemical shift in solution.
[32]
1
340.4 ppm, and compare acceptably well with the calcu-
(IDipp)Si=Si(IDipp), which can be rationalized by the
lated values at the B3LYP/6-311G** level of theory
successive replacement of the PMes* fragment by its electro-
[21]
[36]
(
Table 1). The large span value is a salient spectroscopic
positive pendant Si(IDipp).
Given the known relation
[33]
feature of compounds with E=E bonds. A comparison of
between the HOMO energy and the redox potential for
[33b]
[37]
the experimental d values of 1 with those of P Mes *
oxidation of isostructural compounds, an increase of the
ii
2
2
suggests that the neighborhood to the more electropositive Si
atom predominantly influences the in-plane tensor compo-
nent d22 along the Si=P bond axis, and the out-of-plane tensor
HOMO energy in the series P Mes* !(IDipp)Si=PMes*!
2
2
Si (IDipp) was expected to have a major impact on the
2
2
oxidation potential of these compounds. This was confirmed
by comparative cyclic voltammetric studies in 1,2-difluoro-
benzene at ambient temperature. In fact, the cyclic voltam-
mogram of 1 displays a reversible wave for the oxidation of
[34]
component d pointing in the direction of the SiꢀP p-bond.
3
3
3
1
The most significant contribution to the deshielding of the
P
nucleus comes as in P Mes*2 from the in-plane tensor
2
[
21,38]
component d11 pointing in the direction of the PꢀC
1 (Figure 4).
Oxidation of 1 occurs at a half-wave
Mes*
bond. Its large positive value can be attributed to the low
HOMO(n )–LUMO(p*) gap of 1 of 3.43 eV, which is similar
+
[21]
with that for P Mes* (DE = 3.33 eV; Figure 3).
2
2
HOMO–LUMO
Comparative quantum chemical calculations of 1, Si2-
(
IDipp) , and P Mes* reveal the same number, symmetry
2
2
2
Figure 4. Single-scan cyclic voltammograms of 1 in 1,2-difluoroben-
zene at different scan rates in the potential range ꢀ350–250 mV;
+1/0
reference electrode: 0.4m [Fe(C Me ) ] /0.1m N(nBu) PF /1,2-
5
5
2
4
6
C H F .
6
4 2
potential E1/2 of ꢀ53 mV, which lies in between that for the
reversible one-electron oxidation of Si (IDipp ) (E
=
1/2
2
2
[
38]
ꢀ
794 mV) and the irreversible one-electron oxidation of
P Mes * (E = 1411 mV; v = 100 mVs ) under the same
ꢀ
1 [39]
2
2
1/2
21,40]
[
conditions.
The large increase of the oxidation potential
by 2.2 Vupon moving from Si (IDipp) to P Mes* reflects the
2
2
2
2
large calculated difference of 2.27 eV between the HOMO
energies of the two compounds.
Figure 3. Selected frontier Kohn–Sham orbitals (B3LYP/6-311-G**/RIJ-
COSX/COSMO(n-hexane)) of Si (IDipp) (top), 1 (middle), and
Comparative UV/Vis studies of 1, Si (IDipp) , and
2
2
2
2
ꢀ3
P Mes*2 provide additional evidence for the electronic
2
P Mes* (bottom) (isosurface value 0.05 ebohr ) and their respective
2
2
energies in eV.
analogy of the three double bond systems (Figure 5). In
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3
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1
.68, indicating a strong covalent SiꢀP interaction. This was
further confirmed by the Si=P bond cleavage energy (BCE) of
2
ꢀ
1
70 kJmol , which is reduced to a ZPVE corrected bond
0
ꢀ1
dissociation energy D (0) of 222 kJmol upon electronic and
geometrical relaxation of the fragments Si(IDipp) and PMes*
[36]
into their respective triplet ground states. Interestingly, the
energy required for the SiꢀC
bond dissociation (D8(0) =
NHC
ꢀ
1
1
20 kJmol ) to give IDipp and the phosphasilenylidene (Si=
PMes*) (D, Figure 1) compares well with those of SiX -
IDipp) (X = Cl, Br, I: D8(0) = 121–124 kJmol ) suggest-
2
ꢀ1 [13]
(
ing that 1 might act as a phosphasilenylidene transfer reagent
in the presence of a suitable IDipp trapping agent. The
phosphasilenylidene displays according to quantum theory
a Si=P double bond (d(SiꢀP) = 2.157 ꢀ), an acute angle at the
Figure 5. Electronic absorption spectra of Si (IDipp) , 1, and P Mes*
2
2
2
2
in n-hexane. The arrows indicate the positions of the HOMO (n )!
+
LUMO (p* (E=E)) bands.
P atom (Si-P-C
70.608), and a short contact between the Si
Mes*
atom and a Cortho atom of the Mes* substituent (d(Si···C) =
general, a bathochromic shift of the absorption bands is
2.172 ꢀ), leading to an electronic stabilization of the unsatu-
[21]
ꢀ1
observed in the direction P Mes* !1!Si (IDipp) , providing
rated silicon center. It is less stable by 24.5 kJmol than
the silaphosphyne Mes*SiꢁP (C, Figure 1), which features
a linear-coordinated Si atom and a SiꢀP triple bond (d(Siꢀ
2
2
2
2
a rational for the observed color change in solution from
yellow (P Mes* ) over orange (1) to red (Si (IDipp) ). The
2
2
2
2
[
21]
UV/Vis spectrum of 1 displays three absorption bands at l =
00, 378, and 480 nm (Table 2). Deconvolution of the
observed bands, backed up by TDDFT calculations, allowed
P) = 1.968 ꢀ).
3
In conclusion, the isolation and comprehensive character-
ization of the NHC-stabilized phosphasilenylidene 1 corrob-
orates the ability of N-heterocyclic
carbenes to stabilize unprecedented
bonds between the heavier 3p block
elements. Compound 1, which is the
first example of a compound featur-
ing a Si=P bond to a two-coordinate
silicon atom, contains with the Si=P
bond and the Si and P lone pairs
many potential reactive sites for
[
a]
Table 2: UV/Vis absorption bands of 1, Si (IDipp) , and P Mes* .
2
2
2
2
l1 (e)
l2 (e)
l3 (e)
4
4
[b]
(
(
IDipp)Si=Si(IDipp)
IDipp)Si=PMes* (1)
Mes*P=PMes*
348 (1.26ꢃ10 )
466 (1.43ꢃ10 )
378 (1.49ꢃ10 )
340 (3.93ꢃ10 )
523
480
3
4
[b]
300 (9.37ꢃ10 )
4
3
2
283 (1.66ꢃ10 )
460 (4.63ꢃ10 )
ꢀ1
ꢀ1
[
a] l [nm], e [Lmol cm ]. [b] No extinction coefficient could be determined for these shoulder
absorption bands.
further functionalization. Most
inspiring is, however, the perspec-
a full assignment of these bands. The weak band at l = 480 nm
tive to use 1 as transfer reagent of the elusive phosphasile-
nylidene DSi=PMes* taking advantage of the comparably low
stems from the symmetry forbidden HOMO(n )!
+
LUMO(p*(Si=P)) transition. The corresponding band of
SiꢀC
bond dissociation energy. In fact, preliminary studies
NHC
[
41]
P Mes* appears at l = 460 nm, whereas that of Si (IDipp)2
on the reactions of 1 with unsaturated metal complexes
2
2
2
is not visible owing to its low oscillator strength, but is
suggested by deconvolution of the experimental absorption
bands to appear at l = 610 nm, in good agreement with the
substantiate this perspective.
Received: November 20, 2014
Published online: && &&, &&&&
[
21]
results of the TDDFT calculations. Remarkably, the bath-
ochromic shift of this band in the series P Mes* (460 nm)!
2
2
Keywords: isolobal relationship · main-group elements ·
multiple bonds · phosphorus · silicon
(
IDipp)Si=PMes* (1, 480 nm)!Si (IDipp) (610 nm) corre-
2
2
.
lates well with the decreasing HOMO–LUMO gap in the
same direction.
A further insight into the electronic structure of 1 was
provided by a natural bond orbital (NBO) analysis of the
wavefunction of 1, which revealed a high localization of the
molecular orbitals describing the Si=P double bond. Thus, the
s bond NBO features an occupation of 1.90 electrons,
whereas the p bond NBO is filled with 1.92 electrons. Both
orbitals are slightly polarized towards the P atom (62% and
[
1] G. Becker, G. Gresser, W. Uhl, Z. Naturforsch. B 1981, 36, 16 –
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7%, respectively) and are formed mainly from p-orbitals of
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have predominant s character and show an occupation of 1.95
electrons and 1.88 electrons, respectively. The moderate
polarization of the Si=P bond and the high occupation
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numbers of its NBOs lead to a high Wiberg Bond Index of
33, 170 – 178.
4
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 7
These are not the final page numbers!

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[30] The P NMR chemical shifts of phosphasilenes (R Si=PR)
3
1
2
[
depend strongly on the P-substituent and range from ꢀ33 ppm
(Ref. [28i]) to + 389.3 ppm (Ref. [28j]). Interestingly, the ylidic
phosphasilene LSi=PH (L = CH(C=CH )CMe(NDipp) ) shows
2
2
3
1
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14] A. C. Filippou, O. Chernov, B. Blom, K. Stumpf, G. Schnaken-
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an exceptional shielded P NMR signal at d = ꢀ293.9 ppm
1
( J(P,Si) = 186.4 Hz) (Ref. [28m]).
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1
[
[34] The principal axis system of the P magnetic shielding tensor in
the molecular frame of 1 was calculated using EFGShield2.4 (S.
Adiga, D. Aebi, D. L. Bryce, Can. J. Chem. 2007, 85, 496 – 505)
and is depicted in the Supporting Information.
7
Pakulski, N. C. Norman, Polyhedron 1987, 6, 915 – 919; c) M.
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[35] The isolobal analogy between Si (IDipp) and diphosphenes was
2
2
[
21] The synthesis and full characterization of 1, including the
solution and solid-state NMR spectra, cyclic voltammograms,
results of the quantum chemical calculations, and comparisons of
the experimental and the calculated UV/Vis spectra of 1,
Si (IDipp) , and P Mes* are given in the Supporting Informa-
mentioned recently without justification: D. Himmel, I. Kross-
ing, A. Schnepf, Angew. Chem. Int. Ed. Angew. Chem. Int. Ed.
2014, 53, 370 – 374; Angew. Chem. 2014, 126, 378 – 382.
[36] Both fragments have a triplet ground state with a ZPVE
ꢀ
1
corrected singlet–triplet gap of ꢀ56.3 kJmol (PMes*) and
2
2
2
2
ꢀ
1
tion.
ꢀ30.6 kJmol (Si(IDipp)) calculated at the B3LYP/6-311G**
[
22] S. Inoue, W. Wang, C. Prꢂsang, M. Asay, E. Irran, M. Driess, J.
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23] H. Cui, J. Zhang, C. Cui, Organometallics 2013, 32, 1 – 4.
Angew. Chem. Int. Ed. 2015, 54, 1 – 7
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
F
F
+1/0
[
38] Oxidation of 1 by [Fe(C Me ) ]B(Ar ) (Ar = 2,4,6-(CF ) C H )
[40] The half-wave potentials are given versus the [Fe(C Me ) ]
5 5 2
5
5
2
4
3
3
6
2
occurs rapidly in fluorobenzene at ꢀ408C. Attempts to isolate
redox couple, which was shown to be a superior reference
F
+1/0
the radical salt [(IDipp)Si=PMes*]B(Ar ) were unsuccessful
standard for potentials to the [Fe(C
H
5
)
]
redox couple: I.
4
5
2
owing to its thermolability. In comparison, the green radical salt
Noviandri, K. N. Brown, D. S. Fleming, P. T. Gulyas, P. A. Lay,
A. F. Masters, L. Phillips, J. Phys. Chem. B 1999, 103, 6713 – 6722.
For comparison reasons, the half-wave potential of the [Fe-
F
[
Si (IDipp) ]B(Ar ) was isolated and fully characterized: “The
2
2
4
Reactivity of Molecular Disilicon(0)”: A. C. Filippou, O. Schie-
mann, M. Straßmann, A. Meyer, G. Schnakenburg, M. I. Arz,
The 17th International Symposium on Silicon Chemistry (ISOS
XVII), Berlin 3 – 8 August 2014.
+
1/0
(
C H ) ]
redox couple was determined under the same
5
5 2
+1/0
conditions and found to be + 562 mV vs. the [Fe(C Me ) ]
5
5 2
redox couple.
41] The UV/Vis spectrum of the diphosphene P Mes* was reported
[
[
39] P Mes* also undergoes a reversible one-electron reduction at
2
2
2
2
in CH Cl and analyzed by quantum theory. The results compare
E1/2 = ꢀ1836 mV, in full agreement with previous reports, which
2
2
ꢀ
well with those described herein: a) see Ref. [26]; b) D. V.
Partyka, M. P. Washington, T. G. Gray, J. B. Updegraff III, J. F.
Turner II, J. D. Protasiewicz, J. Am. Chem. Soc. 2009, 131,
have shown that the radical anion [P Mes* ] can be generated
2
2
chemically and by bulk coulometry in THF solution: a) B.
Cetinkaya, P. B. Hitchcock, M. F. Lappert, A. J. Thorne, H.
Goldwhite, J. Chem. Soc. Chem. Commun. 1982, 691 – 693;
b) A. J. Bard, A. H. Cowley, J. E. Kilduff, J. K. Leland, N. C.
Norman, M. Pakulski, G. A. Heath, J. Chem. Soc. Dalton Trans.
10041 – 10048.
[
42] CCDC 1037826 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
1987, 249 – 251.
6
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Communications
Main-Group Multiple Bonds
An NHC-stabilized phosphasilenylidene
was obtained from SiCl (IDipp) (IDipp=
2
D. Geiß, M. I. Arz, M. Straßmann,
G. Schnakenburg,
1,3-bis(2,6-diisopropylphenyl)imidazolin-
2-ylidene) and LiP(Mes*)(TMS) (Mes*=
2,4,6-tBu C H ). The compound was
A. C. Filippou*
&&& — &&&
3
6
2
characterized by various experimental
and theoretical methods and compared
with those of the isolobal congeners
(IDipp)Si=Si(IDipp) and Mes*P=PMes*.
Si=P Double Bonds: Experimental and
Theoretical Study of a NHC-Stabilized
Phosphasilenylidene
Angew. Chem. Int. Ed. 2015, 54, 1 – 7
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!